Advanced wireless communication systems, such as third generation (3G) high speed downlink packet access (HSPDA) systems, often transmit a common pilot channel (CPICH) signal that is used to estimate the quality of a communication channel. One of the typical methods that are used to estimate channel quality includes estimating the SIR of a pilot signal. In packet based systems, the received power level may change often as packets are transmitted at different power levels. An automatic gain control (AGC) circuit is used at a receiver in a wireless transmit/receive unit (WTRU), (i.e., a mobile station), to react to the changes in the power level by adjusting the gain.
In a conventional wireless communication system which generates a pilot signal, time varying gain changes occur on the pilot signal. Thus, the pilot signal becomes non-stationary which results in making it more difficult to estimate the true SIR of the signal. Typically, the true SIR will be underestimated. When the measured SIR is lower than the actual SIR is indicated to the system that the channel quality is worse than it actually is, thus leading to lower throughput and inefficient use of radio resources.
When a 3G HSDPA system has large total transmit (Tx) power variations, as is possible when some subframes carry packets while other subframes do not, the AGC circuit will adapt to the new power level at a rate that depends on implementation of the AGC circuit, while the transmitted pilot power has not changed. Therefore, the pilot symbols which are computed by despreading pilot chips will have mean values and variances that change with time, causing the AGC gain to change. This distorts the accuracy of the SIR computation, as shown in FIG. 1. Because the gain varies during the SIR measurement period, the SIR estimate is often inaccurate when compared to true SIR.